Engine for self-traveling has been installed in construction machine such as hydraulic excavators and the like, the engine operating as a power source to drive hydraulic pump and supply each hydraulic actuator such as rotating actuators, boom cylinders, arm cylinders etc. with hydraulic oil discharged from the hydraulic pump, to drive each portion. However, known construction machine using engine as power source suffer from problems in that they burn too much gas due to large load fluctuation and inordinate burden to the engine, and that they generate noise and/or exhaust gas emissions. Accordingly, hybrid construction machine containing motors driven by combination of generator and electrical energy storage unit (battery) have been developed to overcome the foregoing deficiencies. As examples of the hybrid construction machine, there have been provided a series system one disclosed in Japanese Patent Laid-Open Publication No. 2000-283107 and a parallel system one disclosed in Japanese Patent Laid-Open Publication No. Hei 10-42587/Laid-Open Publication No.2000-226183 etc.
Generally in either system of the hybrid construction machine above, voltage between battery terminals has been measured and the battery state-of-charge SOC has been calculated based on the measurement results. Then, generator has been operated when the state-of-charge SOC become a specified value or less, while the generator has been halted, or the output of the generator has been reduced when the state-of-charge SOC become the specified value or more. With such control, the state-of-charge SOC of the battery has been kept within a specified range.
However, the foregoing state-of-charge SOC based control have suffered from the problem in that halting generator, or reducing the output of the generator caused drop of efficiencies of engine and generator in case of small load and high state-of-charge SOC. In addition, the foregoing state-of-charge SOC based control in the same case above have also suffer from the problem in that there might be a situation that electric actuators might not be supplied with electric power required for its operation because of time-lag to restart engine once halted, and it was required to increase battery capacity to overcome the problem. Therefore, it was difficult to achieve the conditions simultaneously of supplying appropriate electric power required for electric actuator, controlling battery state-of-charge, operating engine and generator highly efficiently. For example, if it was prioritized to operate engine and generator highly efficiently, battery capacity was resulted in increase in case of small load of electric actuator, and therefore it was difficult to control battery state-of-charge SOC within a specified range. In contrast, if it was prioritized to control battery state-of-charge SOC within a specified range, output power fluctuation due to load fluctuation of engine and generator was resulted in increase, and therefore efficiencies of engine and generator drop.
In addition, charging/discharging over battery capacity in case of low battery temperature would have been likely to cause the performance deterioration of the battery because of battery characteristics that charging/discharging capacity depend on its temperature. Further, controlling only based on battery state-of-charge SOC would have been likely to cause the increase of loss because losses of battery and generator vary with output powers of generator and battery respectively while internal loss of battery varies with input power of the battery.